Overview

Four parathyroid glands are found near the posterior aspect of the thyroid gland. They are small (20-40 mg) and have a beanlike shape.

These 4 glands produce parathyroid hormone (PTH), which helps to maintain calcium homeostasis by acting on the renal tubule as well as calcium stores in the skeletal system and by acting indirectly on the gastrointestinal tract through the activation of vitamin D.

The parathyroid glands have a distinct, encapsulated, smooth surface that differs from the thyroid gland, which is has a more lobular surface, and lymph nodes, which are more pitted in appearance. The color of the parathyroid glands is typically light brown to tan, which relates to their fat content, vascularity, and percentage of oxyphil cells within the glands.
[1] The yellow color may be confused with surrounding fat. A distinct hilar vessel is also present that can be seen if the surrounding fat does not obscure the glands' hila.

The superior parathyroid glands are most commonly located in the posterolateral aspect of the superior pole of the thyroid gland at the cricothyroidal cartilage junction. They are most commonly found 1 cm above the intersection of the inferior thyroid artery and the recurrent laryngeal nerve (see the image below). The inferior parathyroid glands are more variable in location and are most commonly found near the lower thyroid pole of the thyroid.

Development

The parathyroid glands develop from the endoderm of the third and fourth pharyngeal pouches. The thymus is also derived from the third pharyngeal pouch. The inferior parathyroid glands are derived from the dorsal part of the third pharyngeal pouch, and the thymus arises from the ventral part of the third pharyngeal pouch. As the inferior parathyroid glands and the thymus migrate together toward the mediastinum, they eventually separate. In most cases, the inferior parathyroid glands become localized near the inferior poles of the thyroid, and the thymus continues to migrate toward the mediastinum.

The superior parathyroid glands are derived from the fourth pharyngeal pouch and migrate together with the ultimobranchial bodies. The ultimobranchial bodies also develop from the fourth pharyngeal pouch, and, during the fifth week of development, these cells detach from the pharyngeal wall and fuse with the posterior aspect of the main body of the thyroid as it descends into the neck. These cells differentiate into the parafollicular cells (C cells) that secrete calcitonin.
[2] The superior parathyroid glands migrate a shorter distance than the inferior glands, which results in a relatively more constant location in the neck.

Because the superior parathyroid glands travel with the ultimobranchial bodies, they remain in contact with the posterior part of the middle third of the thyroid lobes.

Parathyroid vascular anatomy

The inferior parathyroid gland is supplied by the inferior thyroid artery from the thyrocervical trunk. Studies have shown that in approximately 10% of patients, the inferior thyroid artery is absent, most commonly on the left side. In these cases, a branch from the superior thyroid artery supplies the inferior parathyroid gland.
[3] Inferior parathyroid glands that descend into the anterior mediastinum are usually vascularized by the inferior thyroid artery. If a parathyroid is positioned low in the mediastinum, it may be supplied by a thymic branch of the internal thoracic artery or even a direct branch of the aortic arch.
[4]

The superior parathyroid gland is also usually supplied by the inferior thyroid artery or by an anastomotic branch between the inferior thyroid and the superior thyroid artery. Several studies have indicated that in 20-45% of cases, the superior parathyroid glands receive significant vascularity from the superior thyroid artery. This is usually in the form of a posterior branch of the superior thyroid artery given off at the level of the superior pole of the thyroid.
[5, 6]

Next:

Natural Variants

Migration patterns during embryogenesis may cause the parathyroid glands to exhibit a variation in location. However, there are particular characteristics of migration observed that can help to identify superior and inferior parathyroid glands. The position of the parathyroid glands relative to the recurrent laryngeal nerve (RLN) is very important to differentiate upper from lower glands, as in some cases the craniocaudal distance may be small.
[7]

If a coronal plane is made in the path of the RLN in the neck, then the superior parathyroid glands will be located dorsally, or deeper in the neck, and the inferior parathyroid glands will be more ventral, or anterior to this plane (see image below). The superior gland migration patterns extend to the retropharyngeal, retrolaryngeal, retroesophageal, and posterior mediastinum.
[8]

The location of the inferior parathyroid glands exhibits a greater degree of variability than the superior parathyroid glands. This is due to their migration with the thymus, which descends from the angle of the mandible to the pericardium. Thus, ectopic inferior parathyroid glands can lie anywhere along this path of descent, including the carotid sheath. Approximately 61% of the inferior parathyroid glands are found inferior, lateral, or posterior to the lower pole of the thyroid gland.
[9] The inferior parathyroid glands may also commonly be found in the thyrothymic tract or the cervical portion of the thyroid.
[10]

If the inferior glands fail to separate or separation from the thymus is delayed during their descent, the inferior glands may have ectopic locations within the superior mediastinum (see the image below).

Most common location of superior parathyroid (A) and inferior parathyroid (B) glands; shaded areas detail the most common distribution sites of the glands.

Rarely (0.5-4%), parathyroid glands may be ectopically located within the thyroid gland itself.
[11, 12, 13, 14] An intrathyroid parathyroid gland is defined as being completely surrounded on all sides by thyroid tissue. This intrathyroid localization occurs most likely embryologically due to superior parathyroid gland fusion with the ultimobranchial bodies during development.

When the pharyngeal pouches separate from the pharynx, accessory parathyroid fragments may result and lead to supernumerary parathyroid glands. They are usually found at the level of the lower poles of the thyroid lobes or in the thymus. They can also be found in the middle mediastinum at the level of the aortopulmonary window or lateral to the jugulocarotid axis.
[10]